The invention relates to poly-phase electrical power converters suitable for application at high power levels and, in particular, to parallel resonant converters. The parallel resonant converters of the present invention employ link circuits which produce link voltage pulses having zero and non-zero segments which are independently controlled in duration by controlling the duty cycle of the link voltage pulses with electronic switches, and which limit the maximum voltage of the non-zero segments by voltage clamping devices.
The advantage of resonant converters over hard switched converters is that it is possible to apply significantly increased modulation frequencies due to the reduction of switching losses achieved by soft-switching all semiconductors of the converter. The resulting possibility to apply higher modulation frequencies translates into reduced filtering requirements and, therefore, reduced cost.
In spite of the reported successes in the technology of soft-switching at zero voltage or zero current of the link pulses for transferring power at increasingly higher power levels, it appears that the dominance of parallel resonant converters is hampered by certain major obstacles. In most parallel resonant converters for high power applications, link circuit elements between the electric power source and the load generate link voltage pulses with each pulse consisting of an idle or zero segment and a power transfer or non-zero segment. Conventional parallel resonant converters are in a state of at least a complete half-cycle resonance during the entire power transfer segment. The resulting high peak values of voltages or currents to which the components of these kind of converters are exposed, become excessive with increasing modulation frequency, defeating the cost advantage of the reduced filtering requirements as mentioned before.
The drawback of the conventional parallel resonant converters is mitigated in a kind of converter known as parallel quasi-resonant converters (PQRC) or voltage clamped parallel resonant converters. In this kind of converter use is made of the principle of power transfer through quasi-resonance. The first known patent to apply the principal of quasi-resonance to parallel resonant converters suited for high power applications is U.S. Pat. No. 4,864,483 issued Sep. 5, 1989 to Divan. Similar converters are shown in U.S. Pat. No. 5,038,267 issued Aug. 6, 1991 to DeDoncker and U.S. Pat. No. 5,172,309 issued Dec. 15, 1992 to DeDoncker et al. Basically the link voltage pulse of a PQRC is clamped to some near-constant value during the power transfer segment and is only in a state of resonance during the rise of the link voltage pulse to the clamp voltage level and during the termination of the pulse when it falls to zero. The resultant near-square wave pulses obviously lead to a higher duty cycle than the duty cycle achieved from pulses generated by a full resonant cycle as used in conventional parallel converters. Moreover, since these near-square wave pulses are similar to those generated by conventional hard-switched converters, all components of a PQRC incur voltage and current levels which are less than or comparable to those of conventional hard-switched converters. The DeDoncker patents use soft-switching which may be achieved by the zero voltage crossing detector of U.S. Pat. No. 5,166,549 issued Nov. 24, 1992 to DeDoncker.
A crucial problem to overcome with PQRC's is the lack of possibility to control in a continuous fashion the duration of the idle and power transfer segments of the pulses, i.e. continuous control of the duty cycle is not possible. For a PQRC and a conventional hard-switched PWM (pulse width modulated) converter to achieve the same output accuracy, the modulation frequency of the PQRC would need to be a multiple of that used for the conventional hard-switched converter. The basic reason is the need for the PQRC to use integral-cycle control to shape the output waveforms, i.e. the pulses which have fixed durations of idle and power transfer segments are simply distributed over the multiple output phases or switched between positive and negative polarities. This integral-cycle control is obviously inferior to a control which can exploit the flexibility to control the duration of these segments as indeed is fundamental to well-known control strategies such as PWM and pulse area modulation (PAM) control. PWM can be applied to most kinds of hard-switched converters.
Another serious detrimental effect of the need to apply integral-cycle control is on the efficiency of the converter. The lack of controllability of the duration of the power transfer segment leads to wasting of pulses in order to extract excess energy delivered to the load due to instances when the load demands a duration of the power transfer segment to be shorter than the fixed duration for which the PQRC is designed. Moreover, to minimize the reduction of efficiency during operation at fractional load, controllability of the duration of the idle segment is essential. An attempt to allow control of the duration of the power transfer segment is presented in a paper by Divan, Malesani and Toigo: "A Synchronized Resonant DC Link Converter for Soft-Switched PWM", IEEE Trans. on Ind. Appl., published in September 1993. However, the initiation process of the link voltage pulse is conducted by means of two different high-frequency resonance circuits and as a result, the principle of power transfer through quasi-resonance is compromised. Thus, the desired controllability is achieved at the cost of the lack of voltage clamping capability. The peak value of the link voltage to which the converter components are exposed, can be expected to become excessive during large swings of the load current or as the modulation frequency is increased.